The present invention relates to a process for producing pitch-based carbon fibers and the carbon fibers produced by the process, and more in detail, the present invention relates to a process for producing pitch-based carbon fibers having specific properties comparable to those of polyacrylonitrile(PAN)-based carbon fibers by using naphthalene as a starting material, and pitch-based carbon fibers produced by the process thereof.
The carbon fibers which are now commercially available are classified based on the starting material therefor into (1) the carbon fibers produced from PAN, that is, PAN-based carbon fibers and (2) the carbon fibers produced from a pitch, that is, pitch-based carbon fibers. Since PAN-based carbon fibers are generally superior to pitch-based carbon fibers, particularly in tensile strength, most of high performance carbon fibers having high strength and modulus of elasticity have been manufactured from PAN. However, because of the high price of the starting material and the poor yield of carbonization thereof in the PAN-based carbon fibers, studies for producing the pitch-based carbon fibers having comparable tensile strength and Young's modulus to those of PAN-based carbon fibers by using the pitch which can take advantage to PAN as the starting material, have been carried out, and several processes have been proposed.
For instance, a process for producing graphite fibers having a highly three-dimensional order characterized by the cross lattice line (112) and the lines (100) and (101) in the X-ray diffraction pattern and having an interlayer spacing (d.sub.002) of not more than 3.37 .ANG., an apparent layer size (L.sub.a) of not less than 1000 .ANG. and an apparent layer height (L.sub.c) of not less than 1000 .ANG., has been reported, which process comprises heating a petroleum pitch, a coal-tar pitch or an acenaphthylene pitch at a temperature of 350.degree. to 500.degree. C. for a sufficient time for forming about 40 to 90% by weight of a mesophase in the pitch, thereby preparing a carbonaceous pitch showing non-thixotropy at a spinning temperature and a viscosity of 10 to 200 poise, spinning the thus prepared pitch into fibers, subjecting the thus spun fibers to infusibilization (thermosetting) at a temperature of 250.degree. to 400.degree. C. in oxygen-containing atmosphere, heating the infusibilized fibers to a temperature of not less than 1,000.degree. C. in an inert atmosphere and further heating the thus treated fibers to a temperature of not less than about 2,500.degree. C. (refer to Japanese Patent Application Laid-Open No. 49-19127 (1974)).
As is disclosed in Japanese Patent Application Laid Open No. 49-19127 (1974) hitherto it has been considered that in order to produce the high performance carbon fibers from pitch, the use of a mesophase pitch as the starting precursor material is indispensable, because in the case of melt-spinning the mesophase pitch which has molecular orientation, the molecules of the pitch are easily aligned parallel to the fiber axis. However, because of the high softening point the mesophase pitch, the spinning temperature thereof is also high and there is a weak point that at such a high temperature of melt-spinning, the pitch is thermally unstable. In addition, since the mesophase pitch is a heterogeneous mixture containing the isotropic pitch and the pitch liquid crystal, it has been considered that the homogeneous pitch fibers are hardly obtainable.
In order to solve the above-mentioned weak point, a pitch usable as the starting material for melt-spinning, which is not necessarily optically anisotropic before melt-spinning, however, is excellent in spinning property and transforms into optically anisotropic state after being melt-spun or carbonized, , and a process for producing carbon fibers by using such a pitch are proposed.
For instance, a process comprising (1) melt-spinning an optically isotropic premesophase carbonaceous substance or a pitch-like substance mainly composed of an optically isotropic premesophase carbonaceous substance under the melt spinning conditions, which does not substantially increase the content of the mesophase carbonaceous material, (2) infusibilizing the thus melt-spun fibers and (3) carbonizing the thus infusibilized fibers, thereby transforming the premesophase carbonaceous substance or the pitch-like substance containing the premesophase carbonaceous substance into the optically anisotropic mesophase carbonaceous substance is disclosed (refer to Japanese Patent Application Laid-Open No. 58-18421 (1983)). Further a dormant anisotropic pitch of the atomic ratio
of hydrogen to carbon (H/C) of 0.55 to 1.2, which (1) contains as a component for forming the dormant anisotropy, a substantially quinoline-soluble polycyclic polycondensed ring type hydrocarbon which is obtained by partially hydrogenating polycyclic polynuclear hydrocarbons existing in mesophase pitch, (2) forms, in the molten state thereof, a wholly heterogeneous homogeneous and optically isotropic single phase without substantially forming mesophase and (3) in the case where an orientation along the direction of the external force, is disclosed (refer to Japanese Patent Application Laid-Open No. 57-100186 (1982)).
However, in every case thereof, it has been considered necessary to hydrogenate the pitch as the starting material. In addition, in the former case, there is no concrete example of producing the carbon fibers while using only the premesophase pitch, i.e., the quinoline-soluble pitch as the starting material, and the pitch used in melt-spinning generally contains the quinoline-insoluble component.
Further, in order to solve the above-mentioned weak point, the pitch-based carbon fibers having a preferred orientation (22.degree.) of 30.degree. to 50.degree., an apparent crystallite size (L.sub.c) of 12 to 80 .ANG. and an interlayer spacing (d.sub.002) of 3.4 to 3.6 .ANG., measured by X-ray diffractometry, the tensile strength of not less than 200 kgf/mm.sup.2 and the Young's modulus of 10,000 kgf/mm.sup.2 is disclosed in Japanese Patent Application Laid-Open No. 59-53717 (1984). The carbon fibers disclosed in Japanese Patent Application Laid-Open No. 59-53717 (1984) is produced by the process comprising (1) after refining a coal-based heavy oil such as coal tar, coal tar pitch and liquefied coal; topped crude and vacuum residue, tar and pitch by-produced through heat-treatment of the above residues; oilsand and bitumen, adding a solvent for hydrogenation thereto, (2) heating the thus prepared mixture at a temperature of 300.degree. to 500.degree. C. for 10 to 60 min, (3) further heating the thus treated mixture at a temperature of not less than 450.degree. C. for 5 to 60 min under a reduced pressure, thereby obtaining a premesophase pitch for melt-spinning, (4) after heating the pitch for melt-spinning to a temperature of higher than "the temperature of viscosity change", subjecting the thus heated pitch to melt spinning, (5) after quenching the thus melt-spun fibers, subjecting the quenched fibers to infusibilization at a temperature of 250.degree. to 350.degree. C., and (6) heating the thus infusibilized fibers to a temperature of 1,000.degree. to 1,500.degree. C. in an inert gas.
In general, the mechanical properties of carbon fibers depend on the higher order structure. For instance, in order that the carbon fibers are excellent in Young's modulus, it is indispensable that the carbon fibers have a fiber structure and high degree of orientation thereof. Hitherto, in order to produce the pitch-based carbon fibers of high Young's modulus, it has been necessary to use a mesophase pitch which is obtained by thermally treating a raw material such as tar and pitch and crystallizing the carbonaceous material, a dormant anisotropic pitch or a premesophase pitch as the starting material.
Although the pitch-based carbon fibers produced by any process are superior to the PAN-based carbon fibers in the graphitizability, the former is inferior to the latter in tensile strength and it is yet impossible to offer the pitch-based carbon fibers which have the mechanical properties comparable to those of the PAN-based carbon fibers.
As a result of the present inventors' studies for producing the pitch-based carbon fibers having the excellent mechanical properties such as tensile strength, Young's modulus and elongation at break comparable to or superior to those of PAN-based carbon fibers, the present inventors have found that the carbon fibers obtained by the process comprising (1) producing an optically isotropic pitch of a specified molecular structure and molecular weight by catalytically polymerizing naphthalene at a temperature of not more than 330.degree. C., (2) heating the thus obtained polymeric material at a temperature of 330.degree. to 440.degree. C. while introducing an inert gas thereinto to remove volatile components therefrom, (3) melt-spinning the thus obtained pitch, (4) subjecting the thus spun fibers to infusibilization, (5) carbonizing the thus infusibilized fibers, and (6) subjecting the thus carbonized fibers to heat treatment, are surprisingly provided with a fiber structure in which the carbon network planes are oriented parallel to the fiber axis and exhibit excellent mechanical properties such as high strength and high elongation at break which have not been observed by the conventional pitch-based high performance carbon fibers, and based on the above-mentioned findings, the present inventors have attained the present invention.